The control of environments within enclosures is one of the major uses of energy. Maintaining space temperature by either heating or cooling the air within the enclosure is probably the most common type of this control. For at least the last 20 years, it has been known that significant energy savings are available by changing the set point at which the environmental condition is held to one of lower energy usage during certain intervals of the day or week. For example, during the heating season, an unoccupied space or one where occupants are sleeping may be allowed to reach a much lower temperature than is normally comfortable. During the air conditioning season, the space may be allowed to reach a much higher temperature and humidity when unoccupied. This change in set point from one having relatively high energy consumption to one having relatively low energy consumption during selected intervals during a day is called setback, and these intervals are known as setback intervals.
In order to avoid discomfort immediately after the end of the setback interval because temperature (or other parameter) has not reached its normal level, it is necessary to begin the change from the setback set point to the normal set point before the end of the setback interval. This is simply a reflection of the commonly understood fact that environmental control units such as furnaces, air conditioners, etc. cannot instantaneously change the temperature, humidity, or other controlled environmental condition. In order to make the use of energy saving setback operation acceptable to occupants, it is desirable to complete recovery to the normal condition before occupancy begins, or in the case of nighttime temperature cooldown setback, before the occupants arise in the morning. At the same time, it is undesirable to begin recovery to the normal temperature too soon, as this wastes energy.
In the first years of setback type control, the setback interval was fixed. Since the time required to complete setback varies depending on the thermal or other load on the controlled enclosure, this simple type of setback control began to fall out of favor with users. When there was little load on the control unit during the setback interval, less than the programmed time was typically necessary to complete recovery. On the other hand, when the load on the control system was near maximum, then the fixed recovery interval was insufficient and the occupants, upon returning to the enclosure or arising from sleep, were uncomfortable.
The availability of small, inexpensive microprocessors had a significant impact on the management of setback operation. Typical thermostats now use a microprocessor to program the starting and ending times of setback intervals. The computational and memory capabilities of microprocessors permit automatic weekly and yearly variations in the setback intervals based on predictable day to day changes in occupancy and seasonal changes in environmental loads. For example, if the setback interval starting and ending times were simply based on time of day, week, or year, one could program a setback thermostat to set back temperature in a home during Monday through Friday daytimes when residents are at work or school but not on weekends when occupants are likely to be at home. The night time starting time for the setback interval might well start earlier during the weekdays than during the weekend.
An obvious problem with this type of setback control is that it does not take into account the effect of external conditions on the length of the recovery interval. For example, outside winter temperatures can vary unpredictably in excess of a 40.degree. F. range over a period of a few days or weeks. The resulting change in thermal load has a large impact on the length of the recovery interval required to return the enclosure to the normal or occupancy temperature.
The most sophisticated setback controllers base control of the recovery interval starting time on recent history of difference between the actual recovery time and the programmed recovery time. If the actual recovery time is too late, then it is a good bet to start the next day's corresponding recovery interval somewhat earlier. Conversely, if the actual recovery time is too early, then the next day's recovery interval can be started somewhat later. While this results in relatively accurate recovery interval start times, it does require a relatively large memory in the thermostat's microprocessor for this historical data. If for some reason the microprocessor loses power for even a brief time, then this historical data is lost unless there is an on-board backup power supply. And the ability to process this relatively large amount of data requires the microprocessor to have sophisticated software and a larger program memory as well. Such a microprocessor system is relatively expensive in the current market context, even with the constantly dropping prices of such devices.
The significant constraint on the cost of added capability in a setback controller arises from two different considerations. These are first, the energy savings which may be realized by increased accuracy of controlling recovery. The savings which extending setback by a few minutes realizes does not justify more than a modest increase in the cost of the controller as a whole. Secondly, there is the inconvenience cost arising from the discomfort of the occupants when recovery is not completed by the desired time. If the price differential of high capability versus a low capability controller is not clearly justified by these two factors, then it will not be a commercial success. When the price to the end user of the added features becomes a significant fraction of the value of energy which will be saved by the controller over its projected life, then the features will not appeal to prospective users.
U.S. Pat. Nos. 4,702,413 and 4,702,305 disclose a setback thermostat for use with a heating system having two different heating plants. The time at which recovery is started depends on miss times experienced at the end of previous recovery intervals.
U.S. Pat. No. 4,522,336 teaches control of building temperature including a setback phase. The time at which recovery to the occupancy temperature during the setback interval is started to assure reaching occupancy temperature at the desired time depends on the errors recorded for previous recovery intervals.